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@zaveri • 13 Apr, 2013
Now there is an animal which calls itself a "single cylinder, 4-stroke, water cooled, diesel engine".

It is usually found in the thermal labs of all colleges, and i don't know of what practical use it comes to.

It is always started by hand-cranking, and while a person is cranking this, he has to lift up a small lever called the decompression lever, present near the cylinder head, rotate the crank handle really fast, and then finally release this lever.

as soon as this lever is released, the engine starts roaring to life.

now tell me , what is this decompression lever, and what is its mechanism ?
@SYED BASEERULLAH • 20 Apr, 2013 • 1 like I am not much clear about it, but this is what i had understood

I too had found it in our college Thermal lab.
This is used in conventional engines. The pressure to be build up in the cylinder of IC engine is high, and with the need of such high pressure its really hard for a person to crank it.

To facilitate this decompression lever is placed.During cranking the engine,decompression lever is lifted up thereby creating a vent to the cylinder and person doesn't feels hard to crank it. As soon as it's felt that flywheel had got enough inertia the decompression lever is released. Thus vent is closed and pressure builds up inside the cylinder and because of this pressure ignition occurs and thus the engine gets started (roars to life😛)
@zaveri • 22 Apr, 2013 That seems to make sense.

Anyways what have others got to say.

tagging at the i.c engine expert white_hat
@white_hat • 25 Apr, 2013 • 1 like What SYED BASEERULLAH said is correct. Just want to add that ,when you are pressing the decompression lever its not a vent that is opening up but you are actually keeping the exhaust valve open.
@Ankita Katdare • 31 Oct, 2017 • 1 like Has anyone checked this video out?

@Vinay Mishra • 23 Jan, 2020

In a simple way, 

When we pull the decompression lever, It opens the valve of combustion chamber of engine which leads to low the compression. As result the piston can move more freely in cylinder for starting the engine. When piston begin moving, the decompression lever is released by which the compression returns to combustion chamber and allow motor to start.

This is the mechanism of Decompression valve in IC engine.

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